This invention relates to a magnetic head for magnetic recording and reproducing systems such as video tape recorders, which head utilizes an oxide ferromagnetic material as its core material, and more particularly to the use of an improved reinforcement filling material in this type of magnetic head.
A ferrite core magnetic head is now widely used in magnetic recording and reproducing systems for audio and video informations. In a magnetic head of this type, usually two pieces of core members of an oxide ferromagnetic material generally called ferrite, which may be either a single crystal or a molded and sintered mass, are integrated into a core by a glass-bonding technique with the result that a very thin glass film interposed between the two core members gives the magnetic gap of the head. The magnetic gap is made to have a predetermined width corresponding to the track width of a magnetic tape to which the head is applied by shaping appropriately one or both of the core members in their edge portions with respect to a surface subject to glass-bonding. To ensure the bonding of the two pieces of core members and reinforce the magnetic head so as to preclude breaking or cracking of the integrated core particularly in regions shaped for affording a proper width to the magnetic gap, it is a common practice to fill up depressions which are formed in the integrated core as the result of the shaping with a reinforcement filling material so that the lateral edges of the magnetic gap may be covered with the filling material. Until now, a glass composition (an oxide system) which is similar to the core material in hardness (in a solidified state after melting) is used as the filling material for this purpose.
However, conventional ferrite core magnetic heads of the above described construction suffer disadvantages in the following two respects.
The first problem is the charging of the head during operation at its surface brought into contact with a magnetic tape. A Mn-Zn ferrite as a typical example of oxide ferromagnetic materials useful as the core material has a relatively low resistivity, below 1 .OMEGA..cm for single crystal and of the order of 10.sup.2 .OMEGA..cm for sintered mass, but a glass composition used as the reinforcement filling material has a far greater resistivity such as 10.sup.12 .OMEGA..cm for soda lime glass and 10.sup.14 .OMEGA..cm for lead glass (as volume resistivity). A magnetic tape too has a considerably high resistivity. For example, conventional video tapes exhibit surface resistivities of 10.sup.8 .OMEGA. or greater. Accordingly, rubbing of a magnetic head with glass faces adjacent the magnetic gap against a magnetic tape causes not only the tape but also the rubbing surface of the head to be charged electrostatically. As a result, dust in the atomsphere and fine particles of a magnetic material rubbed out from the tape are attracted to and deposit on the rubbing surface of the head. This causes lowering in the recording or reproducing sensitivity of a recording and reproducing system in which the magnetic head is included. In an extreme case for a video tape recorder, there occurs a lack of reproduced picture frames due to the deposition of dust and or the magnetic material particles on the head surface.
The second problem is the damage of the core of the magnetic head attributable to a difference in thermal expansion coefficient between the core material and the reinforcement filling material. The thermal expansion coefficient and its temperature dependence differ from material to material. Oxide ferromagnetic materials generally exhibit an almost linear increase in their thermal expansion coefficient (per .degree. C.) with a rise in temperature up to considerably high temperatures, for example up to about 700-800.degree. C. for a Mn-Zn ferrite. For glass compositions, however, a nearly linear increase in the thermal expansion coefficient (per .degree. C.) ceases at far lower temperatures (for example, at about 500.degree. C. for a kind of lead glass). Thereafter the rate of increase in the expansion coefficient with temperature exhibits a rise for a while, followed by a decrease in the expansion coefficient with further increase in temperature. Even if the thermal expansion coefficient of a glass composition used as the reinforcement filling material is close to that of the core material at relatively low temperatures, heating of the glass composition together with the core material to a temperature above the softening point of the glass to accomplish filling up of the aforementioned depressions with the glass results in that two kinds of materials of different thermal expansion coefficients are integrated and heated together. From this reason the fabrication of conventional ferrite core magnetic heads frequently suffers from the occurrence of microcracks in the core, particularly at the surface to be contacted with a magnetic tape.